1. Field of the Invention
The present invention relates to an imaging lens to be mounted on an imaging apparatus using an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), a digital still camera, a camera-equipped portable telephone, or a personal digital assistance (PDA).
2. Description of Related Art
In a recent imaging device such as a CCD or a CMOS, miniaturization and increase of pixels are highly advancing. Therefore, also the body of an imaging apparatus, and lenses mounted thereon are requested to be further miniaturized and have a higher performance. An imaging lens to be mounted on, for example, a camera-equipped portable telephone is often configured by three lenses. In order to cope with an increased pixel number, however, the number of lenses tends to increase. In order to realize miniaturization and high performance by a reduced number of lenses, furthermore, an aspherical surface tends to be frequently used. JP-A-2000-180719 discloses an imaging lens of a four-lens configuration. The imaging lens disclosed in JP-A-2000-180719 includes a front group, an aperture, and a rear group, sequentially from the object side. The rear group consists of a negative lens, a positive lens, and a positive lens, sequentially from the object side. In the rear group, there is at least one aspherical surface.
In such an imaging lens, aspherical plastic lenses are often used in order to reduce the cost. When only plastic lenses are used, however, there may be an issue in temperature characteristics. In a plastic lens, the lens shape is easily changed with a temperature change, and an image plane change and the like readily occur. In this case, the issue in temperature characteristics can be solved by replacing a lens element having a large power with a glass molded lens. When an aspherical lens is configured by a glass molded lens, however, the cost is inevitably largely increased. In the imaging lens disclosed in JP-A-2000-180719, the temperature characteristics and the cost are not sufficiently studied, and also the compactness is not sufficiently considered.